Generally, the industry of semiconductor manufacturing involves highly complex techniques for fabricating integrating circuits using semiconductor materials. Due to the large scale of circuit integration and the decreasing size of semiconductor devices, decreasingly small defects, such as a particle, can cause the devices to fail. Defect inspection is therefore critical to maintain quality control. Since the inspection methods are an integral and significant part of the manufacturing process, the semiconductor industry is constantly seeking more accurate and efficient testing methods.
Various inspection systems are used within the semiconductor industry to detect defects on a semiconductor device or wafer. One type of inspection tool is an optical inspection system. In optical inspection systems, one or more radiation beams are directed towards the semiconductor wafer or photomask and a reflected and/or scattered beam is then detected. The detected beam is used to then generate a detected electrical signal or an image, and such signal or image is then analyzed to determine whether defects are present on the wafer.
Lasers are used as light sources in many inspection systems to measure defects on photomasks or wafers. Lasers provide light with high intensity to provide ample sample illumination and can provide a collimated beam of light that can be directed easily through lenses and toward the sample. Additionally, laser sources with a short wavelength may be advantageously used for examining relatively small feature sizes.
One of the downsides, however, of using lasers is that the high spatial and temporal coherence of laser light can cause constructive and destructive interference patterns when viewing images of the sample being inspected. The images can be obscured by interference effects such as the edge ringing evident in a coherent image, which can hide detail near any edges in the image, or by speckle, which appears as a nonuniform illumination of the object being imaged. Comprehensive discussions about interference effects such as edge ringing and speckle phenomena can be found in “Fourier Optics”, by J. W. Goodman, McGraw-Hill, and “Statistical Optics”, also by J. W. Goodman, Wiley-Interscience.
These deleterious image effects can be improved by reducing the spatial coherence of the laser light that is used to illuminate the object being imaged. One conventional technique of providing partially incoherent laser light involves the use of a rotating diffuser. A rotating diffuser typically consists of a rotating ground-glass screen that is introduced into the path of the laser beam before it reaches the object being imaged. The rotating diffuser introduces random phase variations into the incident laser beam, thereby introducing spatial incoherence to the beam. As the diffuser rotates, a detector can collect images of the object from independent views or perspectives. The detector, in turn, can integrate or add the independent inspection views to effectively synthesize a uniform illumination of the object being imaged that is relatively free of speckle.
In certain applications, it is frequently required that the inspection system have configurable illumination and imaging designs. The illumination and imaging configuration will be set to optimize the capture of different characteristics of defects or defect types. That is, different illumination and imaging configurations are more suitable for different types of defect inspections. Two broad categories of inspection configurations include bright field and dark field inspection. In general, the illumination and collection beam profiles are adjusted to achieve different inspection modes. In other words, different portions or angles of the incident or collection beam are blocked or transmitted.
For a dark field inspection, a portion of the illumination beam profile is typically blocked so that only a portion of the available illumination, for example a ring of illumination, is passed through to the wafer. A corresponding portion of the collection beam profile is then blocked so that only scattered light is collected. That is, blocked portions of the illumination beam correspond to unblocked portions of the collection beam, while unblocked portions of the illumination beam correspond to blocked portions of the collection beam. Both the illumination and the collection adjustments implement binary masks. That is, portions of the beam are totally blocked by the mask, while other portions of the beam pass unimpeded through the mask. Other types of illumination profiles include annular, dipole, and quadrapole illumination profiles. In general, specific illumination profiles are used to enhance or optimize the imaging quality of specific types of features. By way of examples, a quadrapole illumination profile may be used to image a field of contacts, while a dipole profile may be used to image a vertical line.
Unfortunately, a rotating diffuser typically produces a scattered pattern that has a Gaussian or non-uniform shape at the system pupil. For instance, the intensity may be significantly greater at the pupil center than at the pupil edges. Consequently, it would be difficult to implement various illumination profiles with a rotating diffuser, as opposed to a flatter illumination profile. Additionally, when blocking any of the center portion of the Gaussian shaped profile, a significant amount of light is unused. Thus, this type of arrangement does not efficiently utilize the light.
There is a need for improved and light-efficient mechanisms that provide varying illumination patterns in an incident beam directed at a sample, while reducing effects caused by coherence, such as speckle effect, in the incident beam.